Makeshift classrooms have been set up at flood-damaged Bulli High School as a mammoth clean-up operation continues.

Year 11 and 12 students returned to lessons on Wednesday in parts of the school left untouched by the floodwaters, which swept through the school in the early hours of Tuesday morning during heavy rain.

However, junior students were asked to stay away from the school for the rest of the week to aid the clean-up effort.

Teachers and staff are readying for a challenging week as the full student population - more than 800 children - return on Monday to a reduced number of usable classrooms.

School principal Lou Tome said some classes would be taught in the school library and hall until damaged rooms were repaired.

Administration staff have been moved to the library, as have students from the badly damaged special education building.

Mr Tome said he was satisfied with the speed of the repair arrangements made by the Department of Education.

He also supported an inquiry - by local and state governments - into the cause of flooding to the area, adding: "The school has drainage, but no drainage was going to cope with the amount of water that came down."

Touring the school on Friday, Minister for the Illawarra John Ajaka said the damage was worse than he had anticipated.

Floodwaters had covered the parquetry flooring in the industrial arts and old hall buildings, causing pieces to swell and pop out of formation.

The entire floors will be ripped up; meanwhile, metres of waterlogged carpet are being stripped from the administration, senior studies, special education and music rooms.

Four photocopiers, welding machines, a plasma cutter and numerous musical instruments are among the inventory of damaged items still being written up by staff. At lunchtime Friday, a drying machine was operating in the badly flooded music rooms, where a shallow puddle of water remained.